1. Field of Invention
The instant invention relates generally to the accurate determination of relatively low actinide pollution in water by solid state track recorder (SSTR) metrology, and more particularly to such determination carried out in small individual sample cells wherein a fluidic sample constitutes an asymptotic fission source for a contained SSTR.
2. Background and Description of Prior Art
Hazards created by the leaching of actinides into water have been an ever-present problem in the nuclear energy field since its inception. Principal concerns involve such leaching into the ground water of the ecosystem to cause long-lived pollution, and the leaching of such elements into the fluidic systems of light and heavy water reactors from fuel elements. Various monitoring systems to detect such intrusions have heretofore become known, but in general the known methods have been complex, costly, time-consuming in use, and generally have not provided adequate sensation of relatively small amounts of actinide contamination within sufficiently short time periods to meaningfully predict such contamination. Our invention seeks to provide both apparatus and a process to overcome these problems.
The chief mechanism for pollution of the ecosystem by man generated nuclear activities is by dispersement from areas of radioactive concentration, principally from present day waste disposal sites. Pollutants of primary concern are the long-lived actinide elements which represent the most significant hazards, and their chief dispersal mechanism is commonly groundwater about the nuclear concentration sites. The two primary fissile nuclides that exist in all sites, whether power reactor plants, fuel fabrication facilities, fuel reprocessing plants, or spent fuel storage areas, are U-235 and Pu-239. Measurement of concentrations of these isotopes in ground water may be used to determine contamination release generally both quantitatively and qualitatively, and has become a standard for so doing.
Similarly in light or heavy water reactors, fuel elements may leak because of imperfections in manufacture, radiation induced damage, corrosion, or the like. It is crucial in such structures that leaking fuel elements be identified and replaced as soon as possible, prior to any release of radioactivity and fuel into the water moderator of a reactor. This determination may be approached in the same fashion as environmental pollution.
In any assay method for making the required determinations of contaminant concentrations, there must be high sensitivity to provide appropriate early warning of any actinide release and thereby afford necessary time to identify and resolve the specific problem causing the release. Since time is a crucial factor in addressing and resolving such release problems, the method of highest sensitivity gives the earliest possible warning and hence, the greatest possible protection. The instant invention discloses a process of high sensitivity for measurement of U-235 and Pu-239 contamination in water by using a fission track method for making such determination, with significantly higher sensitivity than known conventional nuclear metrology methods.
Our metrology method uses fission track measurements in solid state track recorders (SSTRs) to assay uranium and plutonium in ground water by reason of the fissionability of those elements. The use of SSTRs to determine radioactive mass is well known and such devices in their essence have heretofore been used to determine actinide contamination in biological water samples, especially by R. P. Larsen and R. D. Oldham, as evidenced in various of their writings and publications known in the nuclear literature. The methods disclosed by Larsen and Oldham, however, are both complex and costly. They involve essentially the extraction of plutonium from biological materials by chemical separation and after separation, electro-deposition of the material to create a plutonium fission deposit which is then placed adjacent a SSTR of the traditional type and irradiated in a known thermal neutron flux for a known period of time. Larsen and Oldham use a polycarbonate polymer type SSTR (Lexan) with typical thermal neutron flux of 2 * 10.sup.13 neutron/cm.sup.2 /sec for a twenty-four hour exposure duration. The plutonium present in the sample is then determined from the observed fission track density, the measured thermal neutron flux, the known thermal neutron fission cross-section of plutonium and the measured efficiencies for
a) extraction of plutonium from the chemical separation process, PA1 b) plate out of the plutonium in the electro-disposition process, and PA1 c) fission track counting in the Lexan SSTR.
Throughout this process, great care must be exercised to prevent contamination by naturally occurring actinides which are ubiquitous at the levels of concentration of interest. Contaminations from natural actinides in apparatus, material, reagents and laboratory environments are sufficient to totally compromise the measurements involved and constitute the controlling factor in determining applicability of the Larsen and Oldham process.
Our invention provides a sampling process to determine actinide pollution in water that uses SSTR assaying methods, but applies those methods in quite a different fashion and with different apparatus to distinguish it from this known prior art.
Our invention firstly provides a simple, inexpensive quantitative early warning process to determine whether actinide pollution has occurred, rather than a more complex and costly qualitative process to determine various parameters of any such pollution. Only after determination that actinide pollution exists is there justification to make the expenditures and expend the efforts required to determine isotopic composition of the pollution using the Larsen and Oldham method or some similar process. Our process is of high sensitivity to allow determination of actinide pollution in water well below maximum permissible concentrations (MPC) as presently set by regulatory bodies. This sensitivity is necessarily required for our invention to fulfill its purposes to provide sufficiently advanced warning to allow time to implement more costly and complex separation-type assay methods and to allow correction of the causes of the problem in the first instance.
Our process allows the use of relatively small sample cells of particular configuration that do not require removal of or external contact with a sample throughout the entire metrology process, after placement of a sample within the sample cell. This provides multiple benefits over known processes in that no actinide contamination is introduced by intermediate stages of processing and that substantial numbers of the smaller sample cells may be simultaneously irradiated in the same thermal neutron fluence. The lowering of potential for sample contamination by naturally occurring actinides is quite material in our process to provide the accuracy and sensitivity that it attains. Similarly, simultaneous processing of a plurality of samples materially effects our process by substantially reducing the time period required to irradiate a given number of samples and reduce the cost of so doing. This benefit becomes more important when considered in connection with the limited radiation volumes existing in reactors. Additionally, the small size of our sample cell lowers any potential radioactivity produced by neutron irradiation of the sample cell and thusly lowers any personnel exposure that might arise in post-irradiation handling of the cells.
Additionally, our sample cell creates an asymptomatic SSTR detection configuration with liquid, that is, the water sample itself serves as the asymptotic detection medium. In an asymptotic SSTR detection configuration, the source of fission fragments, which commonly in the past has been a solid fission deposit, is thicker than the range of fission fragments in the source. Consequently, increasing the source thickness will no longer increase the observed fission track density, with other parameters constant, so that the SSTR sensitivity attains a maximum or limiting value called its asymptotic sensitivity. In this configuration, the fission fragment source is infinitely thick, insofar as the SSTR detector is concerned. Asymptotic SSTR detector configurations have heretofore become known, with metallic foils furnishing the asymptotic media, for neutron dosimetry and measurement of spontaneous fission half lives, and various asymptotic values have been deduced for fissile elements, especially metallic uranium and uranium oxide foil. The concept of a liquid as a source medium for an asymptotic SSTR detection configuration, however, has not previously been known nor has this procedure been applied to water samples in environmental site surveys for measurement of actinide content.
The sensitivities that can be attained using an asymptotic SSTR configuration, wherein the water sample itself provides the asymptotic detection medium, are set forth in Table 1. In this table, .lambda. is the fission track density in tracks per square centimeter, .tau..sub.th is the thermal neutron flux in units of neutrons per square centimeter per second used in irradiating the SSTR, T is the duration of the irradiation, and f.sub.a is the atom fraction of fissile nuclide in the water sample. These estimates are conservative since neutron fluences, that is .tau.*T, can readily be achieved that are orders of magnitude greater than those shown.
TABLE 1 ______________________________________ Estimated Track Densities for MPC Levels in Water.sup.a Actinide Isotope Natural Parameter Actinide U-235 PU-239 ______________________________________ MPC (curies/ 3 E - 11 3 E - 11 5 E - 12 cm.sup.3) .sigma..sub.th cm.sup.2 577 E - 24 577 E - 24 714 E - 24 f.sub.a 8.09 E - 09.sup.b 3.55 E - 07 2.03 E - 12 .lambda. (tracks/cm.sup.2) 3.4 E + 06 1.5 E + 08 1.0 E + 03 Number of 8.0 8.0 1.3 Decays.sup.c ______________________________________ .sup.a Assumptions for the neutron irradiation are: .tau. = 2 .times. 10.sup.12 neutrons/(cm.sup.2 /sec) and T = 3600 sec (1 hour). .sup.b Contribution from U235 at a presence of 0.72 weight percent in natural uranium. .sup.c Number of alpha particle decays in one hour in an asymptotic volum of 1 cm.sup.2 * 20 .mu.m = 2.0 E - 03 cm.sup.3.
There is some difficulty in comparing our fission track process with other nuclear metrology methods having the same purpose, as the intrinsic elements and factors comprising the different methods vary widely, making direct comparisons impossible without introducing assumptions. Commonly, MPC levels are expressed in curies/gram or curies/cm.sup.3 of water, which for an unknown mixture of actinides is 3E-14 curies/cm.sup.3 under current standards. The sensitivity of our fission track process, however, is not based on the decay rate of the actinides present, but rather on the atom density of those actinides, so track density based on these comparison values cannot be calculated, since the specific activity (decay rate/gram) of the unknown mixture is not known and one cannot therefore calculate atom density of the unknown actinide mixture in water.
To compare our method, we must first establish a base line for the concentration of naturally occurring actinides and then use an increase above this level as observed by our process to predict actinide pollution. To do this, our process is applied to water samples containing only naturally occurring actinide impurities that are indigenous to the test environment. Making the necessary assumptions to convert the base line value of our process to that of decay standards, we find that the MPC value of 2E-12 curies/cm.sup.3 yields the track density of 2.3E+05 tracks/cm.sup.3 for baseline environmental water samples. This makes our process of substantially greater sensitivity than other nuclear metrology methods and provides orders of magnitude in response for the observation of an early warning signal from actinide pollution.
Our invention differs from the known art not in any one of these features per se, but rather in the synergistic combination of all of them to provide the structures, functions and processes hereinafter specified and claimed.